4km per liter? I'm not sure if that is even possible if all the fuel is getting to the cylinders, perhaps it's leaking out somewhere, or is not getting even burnt completely.
Can you give us any more details about how the car runs? I think that is where we should start, and once we have an idea of what might be wrong with the car, we can proceed to voltage readings etc...
1985 190E 2.3L - a constant project.